Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as an energy source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several small-sized battery cells for each device. On the other hand, middle- or large-sized devices, such as vehicles, use a middle- or large-sized battery module having a plurality of battery cells electrically connected with each other because high power and large capacity are necessary for the middle- or large-sized devices.
When the secondary battery is used as the power source for the medium- or large-sized devices, a plurality of unit cells (secondary batteries) are connected in series or in series/parallel with each other so as to manufacture a battery module(s) providing high power. Consequently, the battery module is generally constructed in a structure in which the plurality of secondary batteries are electrically connected with each other.
Up to now, nickel-metal hydride secondary batteries have been widely used as the unit cells (battery cells) of the medium- or large-sized battery module. Recently, however, lithium secondary batteries have attracted considerable attention as the unit cells (battery cells) of the medium- or large-sized battery module because the lithium secondary batteries have a high energy density and a high discharge voltage.
Preferably, meanwhile, the middle- or large-sized battery module is manufactured with small size and weight if possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle- or large-sized battery module. Especially, much interest is currently generated in the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the weight of the pouch-shaped battery is small and the manufacturing costs of the pouch-shaped battery are low.
The pouch-shaped battery is manufactured by receiving an electrode assembly in a battery case, generally made of an aluminum laminate sheet, and thermally welding the outer circumference of a receiving part while electrode leads, connected to the electrode assembly, are exposed outside.
FIG. 1 is a perspective view typically illustrating a conventional representative pouch-shaped battery. The pouch-shaped battery 10 shown in FIG. 1 is constructed in a structure in which two electrode leads 11 and 12 protrude from the upper and lower ends of a battery body 13, respectively, while the electrode leads 11 and 12 are opposite to each other. A sheathing member 14 comprises upper and lower sheathing parts. That is, the sheathing member 14 is a two-unit member. An electrode assembly (not shown) is received in a receiving part which is defined between the upper and lower sheathing parts of the sheathing member 14. The opposite sides 14a and the upper and lower ends 14b and 14c, which are contact regions of the upper and lower sheathing parts of the sheathing member 14, are bonded to each other, whereby the pouch-shaped battery 10 is manufactured. The sheathing member 14 is constructed in a laminate structure of a resin layer/a metal film layer/a resin layer. Consequently, it is possible to bond the opposite sides 14a and the upper and lower ends 14b and 14c of the upper and lower sheathing parts of the sheathing member 14, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 14a and the upper and lower ends 14b and 14c of the upper and lower sheathing parts of the sheathing member 14 so as to weld the resin layers thereof to each other. According to circumstances, the opposite sides 14a and the upper and lower ends 14b and 14c of the upper and lower sheathing parts of the sheathing member 14 may be bonded to each other using a bonding agent. For the opposite sides 14a of the sheathing member 14, the same resin layers of the upper and lower sheathing parts of the sheathing member 14 are in direct contact with each other, whereby uniform sealing at the opposite sides 14a of the sheathing member 14 is accomplished by welding. For the upper and lower ends 14b and 14c of the sheathing member 14, on the other hand, the electrode leads 11 and 12 protrude from the upper and lower ends 14b and 14c of the sheathing member 14, respectively. For this reason, the upper and lower ends 14b and 14c of the upper and lower sheathing parts of the sheathing member 14 are thermally welded to each other, while a film-shaped sealing member 16 is interposed between the electrode leads 11 and 12 and the sheathing member 14, in consideration of the thickness of the electrode leads 11 and 12 and the difference in material between the electrode leads 11 and 12 and the sheathing member 14, so as to increase sealability of the sheathing member 14.
However, the pouch-shaped battery has a problem in that the mechanical strength of the sheathing member 14 is low, and therefore, a possibility of a short circuit occurring in the pouch-shaped battery due to an external force is very strong. In order to solve this problem, a plurality of battery cells are mounted in a module member, such as a cartridge or a pack case, to manufacture a battery module.
However, the battery module with the above-stated construction still has a possibility of an internal short circuit occurring. Specifically, the mechanical strength of the module member is greater than the sheathing member, made of the laminate sheet, of each battery cell. For this reason, when external impacts are applied to the battery cells of the battery module, the electrode leads, which are somewhat rigid, move toward the laminate sheet and the electrode assembly, which have relatively low strength. As a result, there is a possibility of internal short circuit occurring.
More specifically, when external impacts are applied to the battery cell at one side where the electrode terminals of the battery cell are located, when the battery cell drops with the electrode terminals thereof down, or when external impacts are applied to the battery cell at the other side opposite to the one side where the electrode terminals of the battery cell are located, the somewhat rigid electrode leads moves toward the electrode assembly, with the result that the ends of the electrode terminals are brought into contact with the electrode assembly, and therefore, a possibility of an internal short circuit occurring is increased. Especially when the battery module is mounted in devices, which are exposed to external impacts and vibration, such as electric bicycles and electric vehicles, a possibility of the battery catching fire or exploding due to an internal short circuit is greatly increased, and therefore, a safety of the battery module is seriously lowered.
FIG. 2 is a typical view illustrating a process of an internal short circuit occurring due to the contact between an electrode lead and an electrode assembly in a general battery module.
Referring to FIG. 2, when an external force is applied to a battery cell 100 at one side where an electrode lead 101 of the battery cell 100 is located or when an external force is applied to the battery cell 100 at the side opposite to the one side where the electrode lead 101 of the battery cell 100 is located, the electrode lead 101 is brought into contact with a specific region 201 of a module member 200, such as a cartridge or a pack case, in direct contact with or adjacent to the electrode lead 101. As a result, the battery case, which has a strength lower than that of the module member 200, is deformed, and therefore, the electrode lead 101 moves toward an electrode assembly (not shown) of the battery cell. Consequently, an internal short circuit due to the contact with the electrode lead 101 and the electrode assembly of the battery cell occurs, whereby the battery may catch fire or explode.
Consequently, there is a high necessity for a technology that is capable of fundamentally preventing the occurrence of an internal short circuit due to the movement of the electrode lead caused by external impacts applied to the battery module.